Electrical power systems can be used to provide electrical power to one more loads such as buildings, appliances, lights, tools, air conditioners, heating units, factory equipment and machinery, power storage units, computers, security systems, etc. The electricity used to power loads is often received from an electrical grid. However, the electricity for loads may also be provided through alternative power sources such as fuel cells, solar arrays, wind turbines, thermo-electric devices, batteries, etc. The alternative power sources can be used in conjunction with the electrical grid, and a plurality of alternative power sources may be combined in a single electrical power system. Alternative power sources are generally combined after conversion of their DC output into an alternating current (AC). As a result, synchronization of alternative power sources is required.
In addition, many alternative power sources use machines such as pumps and blowers which run off auxiliary power. Motors for these pumps and blowers are typically 3-phase AC motors which may require speed control. If the alternative power source generates a direct current (DC), the direct current undergoes several states of power conversion prior to delivery to the motor(s). Alternatively, the power to the motors for pumps, blowers, etc. may be provided using the electrical grid, an inverter, and a variable frequency drive. In such a configuration, two stages of power conversion of the inverter are incurred along with two additional stages of power conversion for driving components of the AC driven variable frequency drive. In general, each power conversion stage that is performed adds cost to the system, adds complexity to the system, and lowers the efficiency of the system.
Operating individual distributed generators such as fuel cell generators both with and without a grid reference and in parallel with each other without a grid reference is problematic in that switch-over from current source to voltage source must be accommodated. Additionally, parallel control of many grid independent generators can be problematic.
A combination of power systems may also be configured to supply power for charging electric vehicles.